Hi There,
Kojic Acid is a skin-lightening agent that is derived from mushrooms and other fungi. It is commonly used in Asian countries for this purpose. I know that is can be found in creams but I am not sure about soap, I don't know if it would make it through saponification so maybe someone else can help you out with that detail.
Here is a link I found for you:
http://en.mimi.hu/beauty/kojic_acid.htmlIn a book I have there is a note that it has been found to be a skin sensitizer and irritant just FYI.
good luck!
Erin